Intrafraction motion: Movement of the prostate after initial treatment setup has been termed intrafraction motion. Estimates of the magnitude and frequency of this motion were initially made using continuous Magnetic Resonance (MR) imaging. Padhani et al reported 16% of patients had > 5mm anterior:posterior motion when imaged for 7 minutes with similar results reported in subsequent MR cine studies, Intrafraction motion can be secondary to organ motion such as bladder filling, respiration or moving rectal gas, or can be due to physical patient motion. With the availability of real-time prostate tracking, clinical data is available to quantify the magnitude and frequency of motion. An early report from Kupelian et al using continuous radiotransponder positioning7 described 41% of fractions with >3mm of motion and 15% > 5mm of motion for > 30 seconds. The risk of motion was noted to increase with longer treatment time.